The present invention relates to a process for producing hydrocarbons from CO--(CO.sub.2)--H.sub.2 mixtures, i.e. CO--H.sub.2 mixtures optionally incorporating CO.sub.2 and referred to as synthesis gases. It more particularly relates to the use of a catalyst making it possible to convert the synthesis gas into a mixture of essentially C.sub.5 + hydrocarbons (i.e. having at least 5 carbon atoms per molecule) usable as a fuel or fuel oil.
The expert knows that synthesis gas can be converted into hydrocarbons in the presence of catalysts containing transition metals. This reaction, generally performed at a temperature between 150.degree. and 350.degree. C. and under pressure, is known as the FISHER-TROPSCH synthesis. The catalysts normally used for the transformation of CO--(CO.sub.2)--H.sub.2 mixtures into liquid or gaseous hydrocarbons generally incorporate at least one metal of the VIII group, such as iron, ruthenium, cobalt or nickel.
Products prepared by FISCHER-TROPSCH synthesis in the presence of these metal catalysts have a very broad molecular weight distribution. Thus, only a small proportion of the products obtained falls within the middle distillates range constituted by kerosene and gas oil fractions, the kerosene fraction or fractions being constituted by a mixture of hydrocarbons having boiling points between 140.degree. and 300.degree. C. and the gas oil fraction or fractions being constituted by a mixture of hydrocarbons having boiling points between 180.degree. and 370.degree. during an atmospheric distillation, such as is performed by the Expert on a petroleum crude.
Considerable efforts have been made since 1973 to improve the middle distillates efficiency of processes based on the conversion of synthesis gases. In particular cobalt, which has been known as a constituent of Fischer-Tropsch catalysts since the early works of SABATIER and SENDERENS (J. Soc. Chem. Ind., 21, 504, 1902) and German patents 29, 787 (1913) and 295,202 (1914) has again been used recently as a main constituent of improved catalytic formulations.
These formulations permit the synthesis of essentially paraffinic and linear hydrocarbons. However, a considerable proportion of these hydrocarbons constitutes paraffins having high boiling points, i.e. beyond the range of middle distillates. It is then advantageous to treat these high boiling point hydrocarbons in a hydrocracking process conveniently used in connection with the treatment of heavy fractions obtained from a petroleum crude, in order to improve the overall middle distillate efficiency.
Among the improved formulations referred to hereinbefore, French patent 2,388,781 claims a process for the preparation of hydrocarbons using a catalyst containing 5 to 50% by weight of one or more metals from the iron group, including cobalt and/or 0.1 to 5% by weight ruthenium and 5 to 50% by weight copper and/or zinc, said catalysts being prepared by impregnation. This process makes it possible to prepare hydrocarbons by the catalytic reaction of carbon monoxide with hydrogen, the H.sub.2 /CO molar ratio being below 1.
French patent 2,370,712 describes the use in a hydrocarbon synthesis process of a catalyst having 10 to 75% by weight of one or more metals from the iron group, as well as 1 to 50% by weight of one or more promoters, including alkaline earth or alkali metals, Ti, Zr, Al, Si, Cu, Ag, Ce, etc. This catalyst is prepared by impregnating a porous support with one or more aqueous solutions of salts of the metals of the iron group and promoters.
Austrian patent 46119/85 describes a catalyst active in Fischer-Tropsch synthesis containing (a) cobalt or a cobalt-based material, (b) silica or a silica precursor and (c) a base or an alkaline material, in which the ratio a:b:c is in the range 1:0.1-100:0.1-100. Said catalyst is prepared by hydrothermal synthesis under pressure and at a temperature between 50.degree. and 500.degree. C.
The conversion of synthesis gas into diesel fuels is also described South African patent 855,317. The latter describes catalyst essentially containing cobalt and ruthenium dispersed on a random support to which has optionally been added a promoter chosen from within groups IIIB or IVB, oxides of actinides or lanthanides, ZrO.sub.2 and TiO.sub.2 being preferred, as well as catalysts essentially containing cobalt and a promoter chosen from within the group constituted by Rh, Pt, Pd, Ir, Os, Ag and Au.
These catalytic formulations are prepared by impregnating the support with an organic solution containing soluble salts of the different elements entering into the composition of these catalysts.
French patent application 91/07,634 describes catalysts containing cobalt, at least one additional element M chosen from within the group constituted by molybdenum and tungsten and at least one element N chosen from among various elements, including ruthenium and copper, the elements being dispersed on a support. Their formulation differs rom that of the catalysts according to the invention described hereinafter by the obligatory presence of molybdenum and/or tungsten, which leads to the formation of a larger proportion of olefins and/or to slightly lower activities.
Patent WO 85/04598 describes a synthesis gas conversion catalyst and the process for preparing said catalyst comprising a hydrolysis stage, e.g. of a compound of silicon or aluminum. This catalyst contains at least one metal chosen from within the groups VIa and VIII of the periodic classification of elements. It does not contain a group Ib element. Moreover, the preferred formulations described in this patent are essentially iron-based. The hydrolysis of the compounds must be carried out for a sufficiently long time (1 to 12 hours) to be essentially total, rapid gelling leading to a catalyst in which the various elements are not homogeneously distributed. The hydrolysis medium and the hydrolyzed species (not containing a silica and/or alumina-based matrix and other elements) are then separated, e.g. by evaporation and then the gel is thermally treated at between 100.degree. and 600.degree. C.